In radio frequency identification, a radio frequency identification card couples an analogue signal sent by a card reader, demodulates the data as sent by the card reader via a radio frequency circuit in the card, and sends the data to a digital circuit for processing. The digital circuit sends the processed data back to the card reader via a load modulation circuit, thus the whole communication process is completed. The process in which the data is sent back to the card reader is load modulation. Unsatisfactory load modulation waveforms or load modulation depth would impact data demodulation by the card reader. Therefore, the load modulation circuit is important and crucial which shall has idea modulation waveform and enough load modulation depth under various cases of field intensity.
A load modulation circuit of prior art, as is shown by FIG. 1, is comprised of NMOS transistors MN1-MN3, and a phase inverter INV1. Wherein, the NMOS transistor MN3 is equivalent to a switch, being open during modulation, and closed otherwise, with the DIN being a load modulation signal provided and controlled by the digital circuit. The connection or closure of the NMOS transistor MN3 will have an impact over the antenna signals: when the NMOS transistor MN3 is connected, the signal on the antenna are pulled down to form a trough, the trough by trough signals constituting a load modulation wave which carries data to be finally demodulated by the card reader. The inductors L1, L2, and the capacitor C1 in FIG. 1 constitute a coupling circuit. The input signal IN is coupled to the card end via the inductors L1 and L2, while the load modulation wave from the card end can also be coupled to the card reader end.
Such a structure is advantageous in its simplicity and ease of implementation, with a quite good load modulation waveform and load modulation depth under weak field intensity; its disadvantages are poor load modulation waveform and modulation depth under strong field intensity, incorrect demodulated data by most card readers, or incorrect demodulation. Incorrect demodulation by the card reader will lead to total communication failure.